1. Field of the Invention
The present invention relates to a display device including a corner cube array.
2. Description of the Related Art
A reflective liquid crystal display device for conducting a display operation by utilizing ambient light as its light source has been known in the art. Unlike a transmissive liquid crystal display device, the reflective liquid crystal display device needs no backlight, thus saving the power for light source and allowing the user to carry a downsized battery. Also, the space to be left for the backlight in a transmissive device or the weight of the device itself can be saved. For that reason, the reflective liquid crystal display device is not only effectively applicable to various types of mobile electronic units that should be as lightweight and as thin as possible but also allows the use of a battery of an increased size when a unit including the reflective display device is designed to have the same size or weight as a unit including the transmissive device. This is because the space to be left for a backlight in the transmissive device can be used for other purposes in the reflective display device. Thus, the reflective liquid crystal display device is expected to increase the longest operating time of those units by leaps and bounds.
Also, an image displayed by a reflective liquid crystal display device has a better contrast than an image displayed by a display device of any other type even when the display device is used outdoors in the sun. For example, when a CRT, i.e., a self-light-emitting display device, is used outdoors in the sun, the contrast ratio of an image displayed thereon decreases considerably. Likewise, even a transmissive liquid crystal display device, subjected to low reflection treatment, also displays an image at a significantly decreased contrast ratio when the device is operated in an environment in which the ambient light is much intenser than the display light (e.g., in direct sunshine). On the other hand, a reflective liquid crystal display device increases the intensity of the display light proportionally to the quantity of the ambient light, thus avoiding the significant decrease in contrast ratio. For that reason, a reflective liquid crystal display device can be used particularly effectively in mobile electronic units that are often used outdoors, e.g., cell phones, notebook computers, digital cameras and camcorders.
Even though the reflective liquid crystal display devices have these advantageous features that are very useful in various applications, the reflective devices currently available are not fully satisfactory yet in terms of their contrast ratio in dark places, definition, and full-color and moving picture display capabilities, for example. Thus, the development of more practically useful, reflective color liquid crystal display devices is awaited.
A technique of combining a scattering type liquid crystal display mode and a retroreflector is one of known measures to improve the display performance of such a reflective color liquid crystal display device. A conventional reflective liquid crystal display device of such a type will be described with reference to FIG. 14.
As shown in FIG. 14, the reflective liquid crystal display device 900 includes a transparent front substrate 1, including color filters 9 and a counter electrode 8 thereon, and a rear substrate 2, which is disposed so as to face the front substrate 1. A scattering type liquid crystal layer 3, which switches between a scattering state and a transmitting state, is provided between these substrates 1 and 2.
On one surface of the rear substrate 2, thin-film transistors (TFTs, not shown) as switching elements, a retroreflector 5, transparent pixel electrodes 50 and so on are provided so as to face the liquid crystal layer 3. By controlling the voltage to be applied to the liquid crystal layer 3 by way of the TFTs and pixel electrodes 50, each pixel region of the liquid crystal layer 3 can be switched from its scattering state into its transmitting state, or vice versa.
The retroreflector 5 has a reflective film 5a with a predetermined surface shape, which is covered with a planarized layer 5b. The pixel electrodes 50 are provided on the planarized layer 5b. The predetermined surface shape of the reflective film 5a is defined by a great number of unit elements, which are arranged in a regular pattern at a pitch that is smaller than that of the color filters 9. Each of the unit elements is defined by three planes that are opposed substantially perpendicularly to each other. By using the retroreflector 5 having such a configuration, a light ray that has been incident onto this display device 900 can be reflected back toward its source.
Hereinafter, it will be described with reference to FIGS. 15A and 15B how this reflective liquid crystal display device 900 operates. FIGS. 15A and 15B schematically illustrate the black and white display modes of the display device 900.
As shown in FIG. 15A, while the liquid crystal layer 3 is controlled to exhibit the transmitting state, an incoming light ray 54, which has been emitted from an external light source 52, is transmitted through the liquid crystal layer 3 and then reflected back by the retroreflector 5 toward its light source 52 as indicated by the arrow 60. Thus, the light ray 54 that has been emitted from the light source 52 does not reach the eyes of a viewer 56. In such a state, the image reaching the eyes of the viewer 56 from this display device 900 is the image of his or her own eyes. In this manner, the xe2x80x9cblackxe2x80x9d display mode is realized.
On the other hand, while the liquid crystal layer 3 is controlled to exhibit the scattering state, the incoming light ray 54 that has been emitted from the light source 52 is scattered and reflected by the liquid crystal layer 3 as indicated by the arrows 62 in FIG. 15B. That is to say, the retroreflector 5 reflects the incoming light ray 54 not just toward its light source 52 but also toward many other directions. As a result, a portion of the incoming light ray 54 reaches the eyes of the viewer 56. In this manner, the xe2x80x9cwhitexe2x80x9d display mode is realized.
Unlike a twisted nematic (TN) mode liquid crystal display device, for example, the reflective liquid crystal display device 900, conducting a display operation under such operating principles, can display the colors black and white without using any polarizer. Using no polarizers, this reflective liquid crystal display device 900 is not affected by a decreased optical efficiency, which is normally unavoidable when polarizers are used, and can display a highly bright image thereon. However, to get a high contrast ratio realized by this reflective liquid crystal display device 900, it is important to maximize the retro-reflectivity of the retroreflector 5 and thereby minimize the amount of unwanted reflected light reaching the viewer""s eyes in the black display mode.
A corner cube reflector, which is implemented as an array of corner cubes, is one of known retroreflectors having highest retro-reflectivities. In the corner cube reflector, each of those corner cubes is made up of three planes that are opposed substantially perpendicularly to each other and has a shape corresponding to one corner of a cube. Reflective liquid crystal display devices using a corner cube array of a very small size (which will be herein referred to as a xe2x80x9cmicro corner cube array (MCCA)xe2x80x9d) as their retroreflector are disclosed, for example, in U.S. Pat. No. 5,182,663 and Japanese Patent Application No. 2001-090908 that was filed by the applicant of the present application. The MCCA may be formed by the manufacturing processing step of etching the surface of a crystalline substrate anisotropically (see Japanese Patent Application No. 2001-306052 that was also filed by the applicant of the present application).
The conventional reflective liquid crystal display device including the corner cube reflector, however, includes the transparent pixel electrodes 50 and liquid crystal layer 3 that are spaced apart from the reflective film 5a of the retroreflector 5 as shown in FIG. 14. If there is a distance between the liquid crystal layer 3 with a light modulating function and the reflective film 5a in this manner, then a parallax problem may happen to deteriorate the display performance eventually. A display device including a retroreflector 5 normally has a relatively great allowance to a parallax. However, it is naturally expected that an even higher display quality should be achieved if this parallax could be reduced.
Also, in the display device shown in FIG. 14, the rugged surface of the reflective film 5a of the retroreflector 5 is planarized by the planarized layer 5b. In such a configuration, however, the incoming light ray may be either absorbed into the planarized layer 5b or not reflected back as intended (i.e., scattered) from the interface between the planarized layer 5b and the liquid crystal layer 3. As a result, a bright image cannot be displayed or the contrast ratio may decrease.
Furthermore, to get good display performance realized by a reflective liquid crystal display device including such a corner cube reflector, it is even more important than usual to establish an appropriate relationship between the pitch or arrangement pattern of the corner cubes and that of pixels. For example, U.S. Pat. No. 5,182,663 identified above describes that if the pixel pitch is set greater than the pitch of corner cubes, then an incoming light ray that has been incident onto a given pixel region and then retro-reflected by the retroreflector will not pass its adjacent pixel region on the way back and the display performance can be improved as a result. However, in the display device disclosed in U.S. Pat. No. 5,182,663, the transparent pixel electrodes are also provided over the retroreflector, thus causing the parallax and other problems mentioned above.
As described above, none of the conventional reflective liquid crystal display devices including the corner cube reflector has ever achieved sufficiently good display performance by eliminating the parallax, increasing the brightness, and getting the appropriate relationship satisfied between the arrangement patterns of the corner cubes and pixel electrodes.
In order to overcome the problems described above, preferred embodiments of the present invention provide a display device that realizes even higher display quality by using a corner cube array as a retroreflector.
A display device according to a preferred embodiment of the present invention preferably includes a light modulating medium layer, a corner cube array, and a reflective electrode layer. The corner cube array is preferably provided on one side of the light modulating medium layer and preferably includes multiple corner cubes as its unit elements. The reflective electrode layer is preferably provided on the corner cube array and preferably includes multiple reflective electrodes that are spaced apart from each other and that are used to apply a voltage to the light modulating medium layer. When the display device is viewed from over the corner cube array, an arrangement pattern of the corner cubes preferably matches an arrangement pattern of the reflective electrodes in at least one direction.
In one preferred embodiment of the present invention, the corner cube array preferably includes multiple surrounding corner cubes that are adjacent to edges of the reflective electrodes. Each of the multiple surrounding corner cubes is preferably either a corner cube that is partially overlapped by the edge of associated one of the reflective electrodes or a corner cube that is in contact with the edge of the associated reflective electrode. The reflective electrodes preferably do not cover the lowest-level points of the surrounding corner cubes.
A display device according to another preferred embodiment of the present invention also preferably includes a light modulating medium layer, a corner cube array, and a reflective electrode layer. The corner cube array is preferably provided on one side of the light modulating medium layer and preferably includes multiple corner cubes as its unit elements. The reflective electrode layer is preferably provided on the corner cube array and preferably includes multiple reflective electrodes that are spaced apart from each other and that are used to apply a voltage to the light modulating medium layer. The reflective electrode layer preferably includes a non-reflecting region, which is located between adjacent ones of the reflective electrodes. The corner cube array preferably includes multiple surrounding corner cubes that are adjacent to edges of the reflective electrodes. Each of the surrounding corner cubes is preferably either a corner cube that is overlapped by both associated one of the reflective electrodes and the non-reflecting region or a corner cube that is in contact with the edge of the associated reflective electrode. Each said surrounding corner cube is preferably adjacent to another one of the corner cubes under the associated reflective electrode. The boundary between the former and latter corner cubes preferably includes highest-level points of the two corner cubes. The boundary between the associated reflective electrode and the non-reflecting region is preferably either more distant from the center of the reflective electrode than the highest-level points are or as distant from the center of the reflective electrode as the highest-level points are.
A display device according to still another preferred embodiment of the present invention also preferably includes a light modulating medium layer, a corner cube array, and a reflective electrode layer. The corner cube array is preferably provided on one side of the light modulating medium layer and preferably includes multiple corner cubes as its unit elements. The reflective electrode layer is preferably provided on the corner cube array and preferably includes multiple reflective electrodes that are spaced apart from each other and that are used to apply a voltage to the light modulating medium layer. The reflective electrode layer preferably includes a non-reflecting region, which is located between adjacent ones of the reflective electrodes. The corner cube array preferably includes multiple surrounding corner cubes that are adjacent to edges of the reflective electrodes. Each of the surrounding corner cubes is preferably either a corner cube that is overlapped by both associated one of the reflective electrodes and the non-reflecting region or a corner cube that is in contact with the edge of the associated reflective electrode. Each said surrounding corner cube is preferably adjacent to another one of the corner cubes under the associated reflective electrode. The boundary between the former and latter corner cubes preferably includes highest-level points of the two corner cubes. The highest-level points are preferably not overlapped by the non-reflecting region.
In one preferred embodiment of the present invention, the non-reflecting region preferably extends along a line that connects together the lowest-level points of adjacent ones of the surrounding corner cubes.
In this particular preferred embodiment, the non-reflecting region preferably has a minimum width that is smaller than the width of a contact portion between two adjacent ones of the surrounding corner cubes.
More specifically, the minimum width of the non-reflecting region is preferably at most 1/{square root over ( )}3 of a pitch Pcc of corner cubes as measured along the line that connects together the lowest-level points of the surrounding corner cubes.
In still another preferred embodiment, xcex1/Pcc greater than 2xe2x88x92{square root over ( )}10/2 is preferably satisfied, where Pcc is a pitch of corner cubes as measured along the line that connects together the lowest-level points of the surrounding corner cubes and a is the minimum width of the non-reflecting region.
A display device according to yet another preferred embodiment of the present invention also preferably includes a light modulating medium layer, a corner cube array, and a reflective electrode layer. The corner cube array is preferably provided on one side of the light modulating medium layer and preferably includes multiple corner cubes as its unit elements. The reflective electrode layer is preferably provided on the corner cube array and preferably includes multiple reflective electrodes that are spaced apart from each other via a non-reflecting region and that are used to apply a voltage to the light modulating medium layer. The non-reflecting region is preferably located between adjacent ones of the reflective electrodes. xcex1/Pcc less than 2xe2x88x92{square root over ( )}10/2 is preferably satisfied, where Pcc is a pitch of corner cubes as measured along a line that connects together the lowest-level points of the corner cubes and xcex1 is the minimum width of the non-reflecting region. The non-reflecting region preferably extends along edges of consecutive ones of the corner cubes. The edges of the consecutive corner cubes preferably define a boundary between the consecutive corner cubes.
In one preferred embodiment of the present invention, the centerline of the non-reflecting region that extends along the edges is preferably substantially aligned with the boundary between the consecutive corner cubes.
In another preferred embodiment of the present invention, the display device preferably further includes electrodes, which are provided on the corner cube array so as to be opposed to the reflector electrode layer and which are electrically connected to the reflective electrodes by way of contact holes. The contact holes are preferably provided through concave portions of the corner cube array.
In still another preferred embodiment, the light modulating medium layer is preferably a liquid crystal layer that exhibits a scattering state and a transmitting state.
In yet another preferred embodiment, the reflective planes of the reflective electrodes are preferably substantially parallel to the surfaces of the corner cubes.
Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.